Various materials exist for which a minimal intrinsic attenuation of the order of 10.sup.-2 to 10.sup.-4 dB/km can be predicted in the spectral region comprised between 2 and 12 .mu.m. They are therefore considered to be suited to the fabrication of extremely low loss optical fibers to be used for transmission systems with widely spaced repeaters, operating in the medium infrared range. Materials used to manufacture optical fibers must also have various characteristics apart from the optical properties, namely, high mechanical strength, chemical and structural stability, and low reactivity with the environment.
Among the various materials those whose characteristics more strictly satisfy these requirements are halide glasses and more particularly fluoride glasses having metal fluorides as basic ingredients.
Some of the main fluoride compounds are listed below; they can be used as glass "formers" or as matrix "stabilizers" or "modifiers" to fabricate fluoride glass structures. For such compounds and for their metallic elements and relevant oxides, melting temperature T.sub.f, boiling temperature T.sub.e, and sublimation or decomposition temperature are indicated below, when known.
______________________________________ T.sub.f (.degree.C.) T.sub.e (.degree.C.) ______________________________________ AlF.sub.3 ; Al; Al.sub.2 O.sub.3 1291 subl; 660.2; 2045 --; 2467; 2980 BaF.sub.2 ; Ba; BaO 1280; 725; 1293 2173; 1140; 2000 BiF.sub.2 ; Bi; Bi.sub.2 O.sub.3 727; 271.3; 820 --; 1560; 1890 GaF.sub.3 ; Ga; Ga.sub.2 O.sub.3 800; 29.78; 1900 1000; 2403; -- GdF.sub.3 ; Gd; Gd.sub.2 O.sub.3 --; 1312; -- --; 3000; -- HfF.sub.4 ; Hf; HfO.sub.2 --; 2150; 2812 --; 5400; 5400 LaF.sub.3 ; La; La.sub.2 O.sub.3 --; 920; 2315 --; 3469; 4200 LiF; Li; Li.sub.2 O 842; 179; &gt;1700 1676; 1317; -- NaF; Na; Na.sub.2 O 988; 97.8; 1275 subl 1695; 892; -- PbF.sub.4 ; Pb; PbO.sub.2 855; 327.5; 290 dec 1290; 1744; -- ScF.sub.3 ; Sc; Sc.sub.2 O.sub.3 --; 1539; -- --; 2727; -- TlF.sub.3 ; Tl; Tl.sub.2 O.sub.3 550 dec; 303; 717 --;1457; 875 dec ThF.sub.4 ; Th; ThO.sub.2 &gt;900; 1700; 3050 --; .about.4000; 4400 ZnF.sub.2 ; Zn; ZnO 872; 419; 1975 1500; 907; -- ZrF.sub.4 ; Zr; ZrO.sub.2 600 subl; 1852; 2715 --; 3578; -- ______________________________________
Even though glass structures derived from elements of Group II (Be, Zn, Ba) of the Periodic Table or of Group III (Al, Sc, La, Th) may be used, matrices derived from the elements of Group IV (Hf, Zr) have proved to be particularly suitable for optical transmission in the medium infrared, range of 2 to 6 .mu.m. Fluorohafnate and fluorozirconate glasses, discovered in France in 1976, are in common use and have all the characteristics necessary for a material to be used in the optical telecommunications field.
Nowadays these materials are treated by the same techniques as have from multicomponent glass technology.
Starting from the fluoride compound which has been obtained by chemical reaction of the basic element or its oxide, mixtures are prepared with the various components in the desired concentrations.
Then the annealing, vitrification and possibly purification are effected inside a furnace, in graphite or platinum crucibles. Finally the vitrified material is collapsed into cylindrical rods or in preforms to feed drawing crucibles or to be directly drawn.
All these operations are highly polluting and, above all, do not permit checks on the optical core/cladding interface structure. Hence, for oxide glasses as well as for halide glasses such direct material treating techniques are not suited to fabricate ultra-low loss optical structures losses of (10.sup.-3 to 10.sup.-4 dB/Km), wth loss values practically coincident with minimum intrinsic loss values theoretically possible for such materials.
Owing to the high melting and boiling temperatures of the compounds and elements of the table given above, indirect synthesis techniques such as the traditional CVD processes cannot be used. In fact inorganic compounds of such elements, which are easily vaporized at ambient temperature do not exist.
Easy vaporization at ambient temperature is fundamental in the applications of CVD process to optical fibre preform manufacture, since a high temperature for the compound vaporization not only entails considerable complications for deposition plants but does not allow an accurate check of the reaction ambient so that pollution can occur.